Antifoaming compositions are materials used in the prevention, removal and control of unwanted foam. Foamed fluids are dispersions of air or other gas as the discontinuous phase in a continuous liquid phase. Usually, since air or gas makes up the larger volume portion of such a foam, the bubbles are separated only by a thin liquid film. Unwanted fluid foams are made up of numerous tiny bubbles of a mechanical or chemical origin which are generated within a liquid and which rise and accumulate at the liquid surface faster than they decay.
The fields in which unwanted foams are encountered are very diverse, with problems ranging from unesthetic foams to foams which are hazardous. Foam problems are common in polymerization, paint processing and application, fermentation, sugar-refining, oil drilling and refining, food preparation, paper manufacture, sewage disposal, textile dyeing, adhesive application and conversion of ores refined by flotation. Liquid coolants, hydraulic fluids, lubricants, aviation fuels and gas absorption fluids may foam with undesirable results under conditions of operation. If not properly controlled foam can reduce equipment capacity and increase processing time and expense, as well as cause other disadvantages.
Although foam can be controlled by making basic changes in the process itself, or by using mechanical defoaming equipment, chemical antifoaming compositions having proven to be the most effective and economical. By adding the chemical antifoam compositions to the system, stabilized films are broken, causing the foam bubbles to decay, and thus substantially or completely defoaming the process.
Among the many chemical compositions which are known to be useful for the prevention and destruction of undesirable foams, some of the most effective and versatile anti-foaming agents are silicone fluids. Silicone-based compounds or agents may be used as supplied, as suspensions in solvents or mixed with a portion of a dry ingredient from the foamer formulation.
Emulsions of silicone fluids are also available commercially for use as antifoaming agents. The features which make the use of emulsions desirable include nonflammability, compatibility with aqueous systems, ease of dilution, and effectiveness of these highly dispersed foams of silicones in applications where surface properties are important.
Silicone emulsions are generally made from standard fluids, emulsifying agent, water, and finely divided solids (e.g., silica) which act as carriers for the silicone, increasing the exposed silicone interfacial area and, consequently, the effectiveness of the emulsion as an antifoaming agent. All classes of emulsifiers can be used: anionic, cationic, and nonionic. In most cases, a water-in-oil dispersion is prepared by passing a mixture of silicone fluid, emulsifier, some water, and solid dispersant through a high shear blending device such as a colloid mill or homogenizer. The resulting paste is then dispersed in a larger amount of water with vigorous agitation. The final product is a silicone-in-water emulsion, wherein the silicone fluid may constitute up to 70% of the total emulsified composition. Most silicone emulsions, as sold, contain 10-70% silicone, but are usually diluted to much lower concentration before use. Commercial silicone emulsions are pourable systems of low to moderate viscosity, with good shelf stability and good resistance to phase separation.
The dimethyl silicones, in particular, are especially useful in antifoaming agents because of their low surface tension, inertness and insolubility in water. Moreover, they are useful at low concentrations against a wide range of foamers.
A recently developed class of improved antifoam agents comprises dimethylpolysiloxane compounds containing untreated and/or treated fumed silica (e.g., fumed silica treated with octacyclotetrasiloxane). The general process by which these antifoam compounds are made requires that a mixture of the dimethylpolysiloxane fluid and the untreated and/or treated fumed silica filler is initially heated to about 150.degree. C. in order to disperse the filler. The mixture is homogenized under pressure or milled and cooled. It is then reheated to about 150.degree. C. for a considerable time to insure proper filler wet out. Finally, the mixture is again cooled and transferred to a storage container for subsequent use in antifoam compositions, such as aqueous emulsified antifoam compositions.
It has now been discovered that stable aqueous, emulsified antifoam compositions can be prepared without having to use an antifoam compound. Instead, a silicone fluid and a treated precipitated silica filler are each added directly and separately to the antifoam composition. The present method is less tedious and less time consuming than prior art methods and provides a composition of better defoaming ability and longer shelf stability than aqueous emulsified silicone compositions which employ untreated precipitated silica or treated fumed silica.